It was the first attack to apparently target Hezbollah's stronghold in the south of the Lebanese capital since the outbreak of the two-year conflict in neighbouring Syria, which has sharply heightened Lebanon's own sectarian tensions.
The United States and Russia have proposed an international peace conference to douse a civil war that has killed more than 80,000 people, driven 1.5 million Syrians as refugees abroad and raised the spectre of sectarian bloodshed in the wider region.
Syria's government will "in principle" attend the talks tentatively set for June in Geneva and believes it will be an opportunity to resolve the crisis, Syrian Foreign Minister Walid al-Moualem said during a visit to Baghdad on Sunday.
But in an apparent rebuff of Western calls for President Bashar al-Assad to cede power as part of any deal on transition, Moualem said: "No power on earth can decide on the future of Syria. Only the Syrian people have the right to do so."
The US and Russian foreign ministers, striving to refloat a plan for a political transition in Syria, were due to meet in Paris on Monday to work out the details.
Whether the exiled Syrian civilian opposition will take part in the envisaged peace talks - and be able to negotiate effectively, given their internal divisions and shaky rapport with rebels inside Syria - remains in doubt.
The United States has been prodding Assad's opponents to unite before the conference. But the Islamist-dominated coalition has been hamstrung by power struggles during talks going on in Istanbul aimed at broadening its representation and electing a cohesive leadership.
The talks stalled on Sunday in a factional dispute over proposals to dilute Qatar's influence on rebel forces, with Saudi Arabia angling to play a greater role now that Iranian-backed Hezbollah was openly fighting for Assad.
Some observers have viewed the commitment by Hezbollah to Assad's cause as indicating the Lebanese movement does not see the United States weighing in against it. Asked whether the militia's role might alter Washington's reluctance to arm the rebels, a spokesman for President Barack Obama said on Sunday, "The calculus that the president is making is something that is regularly reviewed and updated ... Our involvement and our assistance to the opposition there has steadily increased."
European Union foreign ministers meet in Brussels on Monday to discuss British and French calls for them to ease an arms embargo in order to help the rebels obtain weapons. Some other EU states oppose the move, at least until after any peace talks.
Conflict afflicting Lebanon
Syria's conflagration has polarised Lebanon, a country of four million, in whose 15-year civil war to 1990 Syria was a major player and where Syrian troops remained until 2005.
Lebanese Sunni Muslims support the mainly Sunni insurgency against Assad, and Shi'ite Hezbollah stands by the president, whose minority Alawite sect derives from Shi'ite Islam.
In Sunday's attack, one rocket landed in a car sales yard next to a busy road junction in south Beirut's Chiah neighbourhood, and the other struck an apartment several hundred metres away, wounding five people, residents said.
There was no immediate claim of responsibility. Brigadier Selim Idris, head of Syria's Western-backed rebel military command, told Al-Arabiya Television that his forces had not carried out the attack.
He urged rebels to keep their conflict inside Syria.
But another Syrian rebel, Ammar al-Wawi, told Lebanon's LBC Television the attack was a warning to authorities in Beirut to restrain Hezbollah. "In coming days we will do more than this. This is a warning to Hezbollah and the Lebanese government to keep Hezbollah's hands off Syria," he said.
Hezbollah leader Sayyed Hassan Nasrallah had declared on Saturday night that his thousands of fighters were committed to the conflict against what he called radical Sunni Islamist rebels in Syria, whatever the cost.
"We will continue to the end of the road. We accept this responsibility and will accept all sacrifices and expected consequences of this position," he said in a televised speech on Saturday evening. "We will be the ones who bring victory."
Though numbering only in the thousands compared to the tens of thousands of troops and many more irregular Syrian militiamen that Assad can draw on, Hezbollah's fighters, seasoned in urban warfare against Israel as recently as 2006, are a potent force.
French Foreign Minister Laurent Fabius condemned the violent spillover into Lebanon. "The war in Syria must not become the war in Lebanon," he told reporters in Abu Dhabi on Sunday.
Until recently, Nasrallah insisted that Hezbollah had not sent guerrillas to fight alongside Assad's forces.
Syrian government forces reinforced by Hezbollah launched an onslaught last week on Qusair, a rebel-controlled town close to the Lebanese border that rebels have used as a crucial supply corridor for weapons coming into the country.
For Assad, taking Qusair would help keep Damascus, the capital, connected to the Alawite coastal heartland and also hinder links between the rebel-held north and south of Syria.
Lebanese authorities, haunted by Lebanon's own civil war and torn by the same kind of sectarian rifts as Syria, have pursued a policy of "dissociation" from the Syrian turmoil.
But Hezbollah is arguably a stronger force than Beirut's government, which has been unable to stem the flow into Syria of Sunni gunmen who support the rebels or of Hezbollah fighters who back Assad. It has also struggled to absorb nearly half a million refugees coming the other way to escape the fighting.
At least 25 people have been killed in Tripoli in the north of Lebanon over the last week in Sunni-Alawite street fighting triggered in part by the battle for Qusair across the frontier.
In Lebanon's Bekaa Valley, residents said three rockets landed on Sunday close to the mainly Shi'ite border town of Hermel, without causing injuries. Rebels have targeted Hermel from inside Syria several times in recent weeks.
Nasrallah's speech was condemned by former prime minister Saad al-Hariri, a Sunni who said that Hezbollah, set up by Iran in the 1980s to fight Israeli occupation forces in south Lebanon, had abandoned anti-Israeli "resistance" in favour of sectarian conflict in Syria.
"The resistance is ending by your hand and your will," Hariri said in a statement. "The resistance announced its political and military suicide in Qusair."
Hariri is backed by Saudi Arabia, which along with other Sunni Muslim Gulf Arab monarchies has strongly supported the uprising against the Iranian-backed Assad.
The extent to which Hezbollah's support for Assad has alienated Sunni Arabs who admired its battles against Israel was demonstrated on Sunday when the foreign minister of Sunni-ruled Bahrain used unusually strong language to call Nasrallah a "terrorist" and said it was a "religious duty" to stop him.